1. Field of the Invention
The present invention relates to a display unit and a method of fabricating the same, and more particularly, it relates to a display unit having a display electrode formed on an insulator film and a method of fabricating the same.
2. Description of the Prior Art
A transmission liquid crystal display unit employing a polycrystalline silicon TFT is known in general. For example, Japanese Patent Laying-Open No. 8-152651 (1996) discloses such a transmission liquid crystal display unit. FIG. 10 is a sectional view showing a pixel part 150 of the conventional transmission liquid crystal display unit disclosed in the aforementioned gazette. The structure of the pixel part 150 in the conventional transmission liquid crystal display unit is now described with reference to FIG. 10.
In the pixel part 150 of the conventional transmission liquid crystal display unit, a liquid crystal layer 103 filled with liquid crystals is formed between opposed transparent insulating substrates 101 and 102. The transparent insulating substrate 101 is provided with a display electrode 104 of a liquid crystal cell. The transparent insulating, substrate 102 is provided with a common electrode 105 of the liquid crystal cell. The display electrode 104 and the common electrode 105 are opposed to each other through the liquid crystal layer 103. An alignment layer 136a is provided between the liquid crystal layer 103 and the display electrode 105, while another alignment layer 136b is provided between the liquid crystal layer 103 and the common electrode 105.
A polycrystalline silicon film 106 defining an active layer of a TFT 141 is formed on the surface of the transparent insulating substrate 101 closer to the liquid crystal layer 103. A gate insulator film 107 is formed on the polycrystalline silicon film 106. A gate electrode 108 is formed on the gate insulator film 107. A drain region 109 and a source region 110 of an LDD structure are formed on the polycrystalline silicon film 106. The drain region 109 of the LDD structure is formed by a low concentration region 109a and a high concentration region 109b. The source region 110 of the LDD structure is formed by a low concentration region 110a and a high concentration region 110b. The drain region 109 and the source region 110 of the LDD structure and the gate electrode 108 form the TFT 141.
The transparent insulating substrate 101 is provided on a portion adjacent to the TFT 141 with an auxiliary capacitor CS formed through the same step as that for forming the TFT 141. A storage electrode 111 of the auxiliary capacitor CS is formed in the polycrystalline silicon film 106 and connected with the source region 110 of the TFT 141. A dielectric film 112 is formed on the storage electrode 111. A counter electrode 122 of the auxiliary capacitor CS is formed on the dielectric film 112. The dielectric film 112, located on an extension of the gate insulator film 107, is identical in structure to the gate insulator film 107 and formed through the same step as that for forming the gate insulator film 107. The counter electrode 122 is identical in structure to the gate electrode 108 and formed through the same step as that for forming the gate electrode 108. Side wall insulator films 113 are formed on the side walls of the counter electrode 122 and the gate electrode 108. Insulator films 114 are formed on the counter electrode 122 and the gate electrode 108.
An interlayer isolation film 115 is formed on the overall surfaces of the TFT 141 and the auxiliary capacitor CS. The high concentration region 110b forming the source region 110 is connected to a source electrode 119 through a contact hole 117 formed in the interlayer isolation film 115. The high concentration region 109b forming the drain region 109 is connected to a drain electrode 118 forming a drain wire through a contact hole 116. An insulator film 120, an SOG film 132 serving as a planarization film and another insulator film 131 are formed on the overall surface of the device including the interlayer isolation film 115, the drain electrode 118 and the source electrode 119. The SOG film 132 serving as the planarization film is held between the insulator films 120 and 131. The display electrode 104 is formed on the insulator film 131.
The display electrode 104 is connected with the source electrode 119 through a contact hole 121 formed in the insulator film 120, the SOG film 132 and the insulator film 131. The aforementioned SOG film 132 fills up steps formed on ends of the auxiliary capacitor CS thereby flattening the surface of the display electrode 104. An aluminum alloy is generally employed as the material for the drain electrode 118 and the source electrode 119. Further, an ITO (indium tin oxide) film is generally employed as the material for the display electrode 104. The display electrode 104, the drain electrode 118 and the source electrode 119 are generally formed by sputtering.
In the aforementioned structure, the SOG film 132 serving as the planarization film is provided for the following reason: If large steps are caused on the display electrode 104, liquid crystal molecules cannot be homogeneously oriented in portions of the liquid crystal layer 103 located on the steps. When the liquid crystal molecules are heterogeneously oriented in the liquid crystal layer 103, the display electrode 104 cannot control light transmission and light interception of the liquid crystal layer 103, leading to a regular light transmission state. In this case, the contrast is lowered on the step portions regularly in the light transmission state. In the step portions, further, the thickness of the display electrode 104 is so reduced that the resistance value of the display electrode 104 is increased or the display electrode 104 is disadvantageously disconnected. In order to flatten the surface of the display electrode 104, therefore, the SOG film 132 is provided between the display electrode 104 and the insulator film 131 as the planarization film.
The term xe2x80x9cSOG (spin on glass) film 132xe2x80x9d generically indicates a film mainly composed of a silicon dioxide formed from a solution prepared by dissolving a silicon compound in an organic solvent. Spin coating is employed for applying the SOG film 132. More specifically, the solution prepared by dissolving the silicon compound in the organic solvent is dripped on a substrate while rotating the substrate. Thus, a coating of the solution is formed thickly on concave portions of steps defined on the substrate due to wiring and thinly on convex, to relax the steps. Consequently, the surface of the coating of the solution is flattened. Then, heat treatment is performed for evaporating the organic solvent and progressing polymerization, thereby forming the SOG film 132 having a flat surface.
The SOG film 132 includes an inorganic SOG film containing no organic component in the silicon compound as expressed in the following general formula (1) and an organic SOG film containing an organic component in the silicon compound as expressed in the following general formula (2):
[SiO2]nxe2x80x83xe2x80x83(1) 
[RXSiOY]nxe2x80x83xe2x80x83(2) 
where n, X and Y represent integers, and R represents an organic group such as an alkyl group or an aryl group.
The inorganic SOG film contains large quantities of moisture and hydroxyl groups, has high hygroscopicity, is fragile as compared with a silicon oxide film formed by CVD (chemical vapor deposition), and readily cracked in heat treatment when its thickness is in excess of 0.5 xcexcm.
On the other hand, the organic SOG film has portions where bonds are closed with alkyl groups or aryl groups and is hence inhibited from cracking in heat treatment, and its thickness can be set to about 0.5 to 1 xcexcm. When employing the organic SOG film, therefore, an interlayer isolation film having a large thickness can be obtained and large steps defined on the substrate can be sufficiently flattened. However, the organic SOG film also contains moisture and hydroxyl groups although the quantities thereof are small as compared with the inorganic SOG film, and has high hygroscopicity.
Thus, the SOG film 132 serving as the planarization film contains moisture and hydroxyl groups, and has high hygroscopicity. The SOG film 132 partially discharges the moisture and hydroxyl groups contained therein due to temperature change or pressure change.
A photosensitive resin insulator film or another coating resin insulator film (a polyimide resin film, an acrylic resin film, an epoxy resin film or the like) can also be employed as the planarization film.
However, the resin insulator film or the organic SOG film, having organic groups in its components, discharges organic gas such as methane due to temperature change or pressure change.
Moisture, hydroxyl groups and organic gas discharged from the planarization film deteriorate the alignment layer 136a and the liquid crystal layer 103 or form bubbles in the liquid crystal layer 103 to cause defective display.
In order to prevent such inconvenience, there is a method of forming a film having a properly transmitting neither hydroxyl groups nor gas and performing treatment for suppressing transmission on the film.
Japanese Patent Laying-Open 8-152651 disclosing the aforementioned conventional structure describes a technique of forming the insulator film 131 on the SOG film 132 by plasma CVD and thereafter performing treatment for improving (modifying) the property of suppressing transmission of moisture and gas on the insulator film 131. This gazette also describes that a silicon oxide film, a silicon nitride film or a silicon oxynitride film is employed as the insulator film 131 and the treatment for modification may be performed by one of the following two methods:
In the first method, ions are implanted into the surface of the insulator film 131 formed by a plasma TEOS film or a plasma oxide film. The implanted ions are prepared from silicon ions, inert gas ions, arsenic ions, phosphorus ions or the like. In the second method, treatment with hydrogen plasma is performed on the surface of the insulator film 131 formed by a plasma TEOS film or a plasma oxide film.
However, the aforementioned method of modifying the conventional liquid display unit has the following problems: When forming the display electrode 104 of ITO, an ITO film must be formed on the overall surface of the insulator film 131 to be thereafter patterned into a desired shape by etching. In this case, the surface of the insulator film 131 is removed or damaged due to the etching for forming the display electrode 104 although the insulator film 131 is modified, and the effect of modifying the surface of the insulator film 131 is disadvantageously lost as a result. Therefore, it is difficult to solve such inconvenience that moisture or the like contained in the SOG film 132 is transmitted through the insulator film 131 to deteriorate the alignment layer 136a and the liquid crystal layer 103 or forms bubbles in the liquid crystal layer 103 to cause defective display after formation of the display electrode 104.
Deterioration of the alignment layer 136a conceivably also results from decomposition of the ITO film forming the display electrode 104. More specifically, the ITO film forming the display electrode 104 is decomposed to form indium and oxygen. Such indium and oxygen conceivably adhere to the surface of the alignment layer 136a, to deteriorate the alignment layer 136a. FIG. 11 shows a contrast ratio at the time of performing an aging test on the conventional liquid crystal display unit employing an ITO film as the display electrode 104. As shown in FIG. 11, the alignment layer 136a is deteriorated with time to disadvantageously reduce the contrast in the prior art.
An object of the present invention is to provide a display unit capable of preventing moisture and gas from penetrating into a liquid crystal layer and an alignment layer also after forming a display electrode and suppressing decomposition of materials forming the display electrode.
Another object of the present invention is to provide a display unit capable of suppressing decomposition of materials forming a display electrode thereby preventing deterioration of an alignment layer.
Still another object of the present invention is to provide a method of fabricating a display unit capable of preventing moisture and gas from penetrating into a liquid crystal layer and an alignment layer from a substrate through an insulator film without losing an effect of the insulator film preventing transmission of moisture and gas during fabrication steps.
A further object of the present invention is to provide a method of fabricating a display unit capable of preventing deterioration of an alignment layer by suppressing decomposition of elements forming a display electrode.
A display unit according to an aspect of the present invention comprises an insulator film formed on a substrate, a display electrode formed on the insulator film and an impurity-introduced layer, formed on the surface of the display electrode and the surface of the insulator film, containing an impurity element having high electronegativity. Examples of the element having high electronegativity are fluorine, oxygen, nitrogen, chlorine, bromine, carbon, sulfur, iodine, serene, hydrogen, phosphorus, tellurium, boron and arsenic. When the insulator film is a silicon oxide film, a silicon nitride film or a silicon oxynitride film, the impurity element having high electronegativity is preferably prepared from fluorine, chlorine, bromine, carbon, sulfur, iodine, serene, hydrogen, phosphorus, tellurium, boron or arsenic.
In the display unit according to the aforementioned aspect, the impurity-introduced layer containing the impurity element having high electronegativity is provided on the surface of the display electrode and the surface of the insulator film thereby improving effects of the insulator film and the display electrode preventing transmission of moisture and gas also after forming the display electrode. Thus, the substrate can be prevented from discharging moisture and gas toward a liquid crystal layer and an alignment layer also after forming the display electrode. Consequently, the liquid crystal layer and the alignment layer ca be effectively prevented from deterioration and defective display resulting from moisture and gas. Further, the surface of an ITO film forming the display electrode is stabilized by forming the impurity-introduced layer containing the impurity having high electronegativity on the surface of the display electrode, whereby the ITO film is inhibited from decomposition. Consequently, the alignment layer can be inhibited from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode. Thus, an excellent contrast can be maintained over a long period.
In the display unit according to the aforementioned aspect, the insulator film preferably includes an insulator film containing an organic component. Thus, the insulator film can be effectively prevented from cracking.
In the display unit according to the aforementioned aspect, the impurity element having high electronegativity preferably includes fluorine. When employing fluorine having the highest electronegativity, the ratio capable of terminating dangling bonds or weak bonds of the insulator film with fluorine is increased when forming the impurity-introduced layer by fluorinating the insulator film and the display electrode while the ITO film forming the display film readily reacts with fluorine. Thus, the effects of the insulator film and the display electrode preventing transmission of moisture and gas can be further improved while the alignment layer can be inhibited from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode. In this case, the impurity-introduced layer is preferably formed on the surface of the insulator film, and preferably includes any of a fluoride layer of a silicon oxide film, a fluoride layer of a silicon nitride film and a fluoride layer of a silicon oxynitride film.
In this case, the impurity-introduced layer preferably includes a first layer, formed on the surface of the display electrode, mainly composed of indium fluoride. Thus, the first layer mainly composed of indium fluoride stabilizes the surface of the ITO film forming the display electrode, thereby suppressing decomposition of the ITO film. Consequently, the alignment layer can be inhibited from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode. Further, the first layer improves the effect of the display electrode preventing transmission of moisture and gas, to be capable of preventing the alignment layer formed on the display film from deterioration resulting from moisture and gas. In this case, the display unit preferably further comprises a second layer, formed on the first layer, mainly composed of carbon fluoride. Thus, the first and second layers can further suppress deterioration of the alignment layer conceivably resulting from decomposition of the ITO film forming the display electrode while further suppressing deterioration of the alignment layer formed on the display electrode resulting from moisture and gas.
A display unit according to another aspect of the present invention comprises an insulator film formed on a substrate, a display electrode formed on the insulator film and a first layer, formed on the surface of the display electrode, mainly composed of indium fluoride.
In the display unit according to the aforementioned aspect, the first layer mainly composed of indium fluoride is provided on the surface of the display electrode thereby stabilizing the surface of an ITO film forming the display electrode, for suppressing decomposition of the ITO film. Consequently, an alignment layer can be inhibited from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode. Further, the first layer improves an effect of the display electrode preventing transmission of moisture and gas, whereby the alignment layer formed on the display electrode can be prevented from deterioration resulting from moisture and gas. Thus, an excellent contract can be maintained over a long period.
The display unit according to the aforementioned aspect preferably further comprises a second layer, formed on the first layer, mainly composed of carbon fluoride. Thus, the first and second layers can further suppress deterioration of the alignment layer resulting from decomposition of the ITO film forming the display electrode while further suppressing deterioration of the alignment layer formed on the display electrode resulting from moisture and gas.
A method of fabricating a display unit according to still another aspect of the present invention comprises steps of forming an insulator film on a substrate, forming a display electrode on the insulator film and introducing an impurity element having high electronegativity into at least a portion of the insulator film not covered with the display electrode after formation of the display electrode.
In the method of fabricating a display unit according to the aforementioned aspect, the impurity element having high electronegativity is introduced into at least the portion of the insulator film not covered with the display electrode after forming the display electrode, thereby terminating dangling bonds in the surface of the insulator film with the impurity element having high electronegativity and replacing weak bonds in the surface of the insulator film with bonds with the impurity element having high electronegativity after forming the display electrode. Thus, a function of preventing transmission of moisture and gas is reinforced at least in the insulator film after formation of the display electrode. Consequently, the substrate can be effectively prevented from discharging moisture and gas toward a liquid crystal layer and an alignment layer after forming the display electrode. Thus, the liquid crystal layer and the alignment layer can be effectively prevented from deterioration and defective display resulting from moisture and gas. The impurity element having high electronegativity is introduced after forming the display electrode so that the surface of the insulator film containing the introduced impurity element having high electronegativity is not treated by etching for forming the display electrode or the like, whereby the effect of preventing transmission of gas is not lost in the surface of the insulator film.
In the method of fabricating a display unit according to the aforementioned aspect, the step of introducing the impurity element preferably includes a step of etching the surface of at least the portion of the insulator film not covered with the display electrode simultaneously with introduction of the impurity element. Thus, the effect of the insulator film preventing transmission of moisture and gas can be ensured. The surface of the insulator film is frequently damaged or suffers from adhesion of foreign matter in the process of forming the display electrode. If the degree of the damage or adhesion of foreign matter is high, introduction of the impurity element having high electronegativity is inhibited or a sufficient effect of preventing transmission cannot be attained. In this case, the surface of the insulator film is etched as described above so that the impurity element having high electronegativity can be introduced after removing damage or foreign matter, thereby ensuring the effect of the insulator film preventing transmission of moisture and gas.
In the method of fabricating a display unit according to the aforementioned aspect, the step of introducing the impurity element having high electronegativity preferably includes a step of exposing at least the portion of the insulator film not covered with the display electrode to plasma containing the impurity element having high electronegativity. Alternatively, the step of introducing the impurity element having high electronegativity preferably includes a step of exposing at least the portion of the insulator film not covered with the display electrode to a radical containing the impurity element having high electronegativity. When employing the plasma or radical in the aforementioned manner, the quantity of the introduced impurity element having high electronegativity and the speed for introducing the same can be effectively increased.
In the method of fabricating a display unit according to the aforementioned aspect, the step of introducing the impurity element having high electronegativity preferably includes a step of exposing at least the portion of the insulator film not covered with the display electrode to gas containing the impurity element having high electronegativity. Alternatively, the step of introducing the impurity element having high electronegativity preferably includes a step of exposing at least the portion of the insulator film not covered with the display electrode to liquid containing the impurity element having high electronegativity. When employing the gas or liquid in the aforementioned manner, the impurity element having high electronegativity can be introduced through a low-priced apparatus, whereby the fabrication cost can be reduced.
In the method of fabricating a display unit according to the aforementioned aspect, the step of introducing the impurity element having high electronegativity preferably includes a step of introducing ions containing the impurity element having high electronegativity into at least the portion of the insulator film not covered with the display electrode. When employing ion implantation, the quantity of the introduced impurity having high electronegativity and the depth of introduction thereof can be readily and precisely controlled.
In the method of fabricating a display unit according to the aforementioned aspect, the insulator film preferably includes an insulator film containing an organic component. When employing such an insulator film containing an organic component, cracking can be reduced.
In the method of fabricating a display unit according to the aforementioned aspect, the impurity element having high electronegativity preferably includes fluorine. When employing fluorine having the highest electronegativity, the ratio capable of terminating dangling bonds or weak bonds of the insulator film with fluorine is increased when forming an impurity-introduced layer by fluorinating the insulator film. Thus, the effect of the insulator film preventing transmission of moisture and gas can be further improved. In this case, the step of introducing the impurity element preferably includes a step of forming any of a fluoride layer of a silicon oxide film, a fluoride layer of a silicon nitride film and a fluoride layer of a silicon oxynitride film on the surface of the insulator film by introducing said impurity element.
In the method of fabricating a display unit according to the aforementioned aspect, the step of introducing the impurity element having high electronegativity preferably includes a step of introducing the impurity element having high electronegativity into both of the insulator film and the display electrode. In this case, the effects of preventing transmission of moisture and gas are improved in both of the insulator film and the display electrode. Thus, the substrate can be prevented from discharging moisture and gas toward a liquid crystal layer or an alignment layer. Consequently, the liquid crystal layer and the alignment layer can be effectively prevented from deterioration and defective display resulting from moisture and gas. The impurity element having high electronegativity is introduced into the display electrode for stabilizing the surface of an ITO film forming the display electrode, thereby suppressing decomposition of the ITO film. Consequently, the alignment layer can be inhibited from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode.
In the method of fabricating a display unit according to the aforementioned aspect, the step of introducing the impurity element having high electronegativity preferably includes a step of fluorinating the display electrode thereby forming a first layer mainly composed of indium fluoride on the surface of the display electrode. Thus, the first layer mainly composed of indium fluoride stabilizes the surface of the ITO film forming the display electrode, thereby suppressing decomposition of the ITO film. Consequently, the alignment layer can be inhibited from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode. Further, the first layer improves the effect of the display electrode preventing transmission of moisture and gas, thereby preventing the alignment layer formed on the display electrode from deterioration resulting from moisture and gas.
In this case, the step of fluorinating the display electrode preferably includes a step of forming the first layer mainly composed of indium fluoride on the surface of the display electrode while forming a second layer mainly composed of carbon fluoride on the first layer by exposing the surface of the display electrode to plasma containing fluorine and carbon. Thus, the first and second layers can further inhibit the alignment layer from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode while further suppressing deterioration of the alignment layer formed on the display electrode resulting from moisture and gas.
The step of introducing the impurity element having high electronegativity may include a step of depositing a first layer mainly composed of indium fluoride on the display electrode by sputtering.
A method of fabricating a display unit according to a further aspect of the present invention comprises steps of forming an insulator film on a substrate, forming a display electrode on the insulator film and forming a layer containing fluorine on the surface of the display electrode.
In the method of fabricating a display unit according to the aforementioned aspect, the layer containing fluorine is formed on the surface of the display electrode for stabilizing the surface of an ITO film forming the display electrode with the layer containing fluorine, thereby suppressing decomposition of the ITO film. Consequently, an alignment layer can be inhibited from deterioration conceivably resulting from decomposition of the ITO film forming the display electrode. Further, the layer containing fluorine improves an effect of the display electrode preventing transmission of moisture and gas, thereby preventing the alignment layer formed on the display electrode from deterioration resulting from moisture and gas. Thus, an excellent contrast can be maintained over a long period.
In this case, the step of forming the layer containing fluorine preferably includes a step of forming a first layer mainly composed of indium fluoride on the surface of the display electrode while forming a second layer mainly composed of carbon fluoride on the first layer by exposing the surface of the display electrode to plasma containing fluorine and carbon. Thus, the first and second layers can further suppress deterioration of the alignment layer conceivably resulting from decomposition of the ITO film forming the display electrode while further suppressing deterioration of the alignment layer formed on the display electrode resulting from moisture and gas. The step of forming the layer containing fluorine may include a step of depositing a first layer mainly composed of indium fluoride on the display electrode by sputtering.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.